Light-absorbing dyes incorporated in silver halide photographic materials can accomplish a variety of goals, including their use as filter dyes, acutance dyes or antihalation dyes.
When present in a non-photosensitive top layer or intermediate layer they typically serve as filter dyes eliminating an unwanted part of the light spectrum of the exposure source. A well-known example is the yellow filter layer usually present in colour photographic materials in order to prevent blue light from reaching the green sensitive and red sensitive layers. Another example is formed by UV-absorbing compounds, usually present in the top protective layer, which prevent photochemical deterioration of the image dyes formed by colour development. Examples of useful UV-absorbers include the cyanomethyl sulphone-derived merocyanines of U.S. Pat. No. 3,723,154, the thiazolidones, benzotriazoles and thiazolothiazoles of U.S. Pat. Nos. 2,739,888, 3,253,921, 3,250,617 and 2,739,971, the triazoles of U.S. Pat. No. 3,004,896, and the hemioxonols of U.S. Pat. No. 3,125,597.
On the other hand light-absorbing dyes when present in the emulsion layer can serve as so-called "acutance dyes" or "screening dyes" improving the image sharpness by reducing the sidewise scattering of light by the emulsion grains.
In a third application light-absorbing dyes act as "anti-halation dyes" improving the image sharpness by diminishing the upward reflection of light by the support into the emulsion layer. For this purpose the dye can be incorporated in an undercoat, being a non-photosensitive layer between the emulsion layer and the support, or it can be incorporated in the base itself, or preferably, it can be present in one or more backing layers of the photographic material.
Apart from conventional photographic materials light-absorbing dyes can be useful for similar purposes in so-called photothermographic materials, such as materials based on the so-called Dry Silver technology originally developed by 3M. It is a catalytic process which couples the light-capturing capability of silver halide to the image-forming capability of organic silver salts, such as silver behenate. Traditionally, silver halide, preferably silver bromide, is formed in situ by reacting silver behenate with bromide ions. The result of this process is the formation of very fine grains of silver bromide, less than 500 Angstroms in diameter and positioned in catalytic proximity to the silver behenate. Exposure to light causes photolytic reduction at the silver bromide crystal (latent image formation) and provides a silver nucleus in position to permit catalysis of the reduction of the organic silver salt to silver metal at an elevated temperature thus producing a visual density. A disadvantage of this technology is that in the non-exposed areas silver halide remains which forms print-out silver on aging thereby increasing the minimal density possibly to an unacceptable level for some purposes. Details on the dry silver technology can be found in U.S. Pat. Nos. 3,457,075, 3,839,049, 4,260,677 and J. Phot. Sci., Vol. 41 (1993), p. 108.
Useful dyes absorbing in the visible spectral region include, for instance, the coloured pigments of U.S. Pat. No. 2,697,037, the pyrazolone oxonol dyes of U.S. Pat. No. 2,274,782, the styryl and butadienyl dyes of U.S. Pat. No. 3,423,207, the diaryl azo dyes of U.S. Pat. No. 2,956,879, the merocyanine dyes of U.S. Pat. No. 2,527,583, the merocyanine and oxonol dyes of U.S. Pat. No. 3,486,897, U.S. Pat. Nos. 3,652,284 and 3,718,472, and the enaminohemioxonol dyes of U.S. Pat. No. 3,976.661. Absorbing dyes can be added as particulate dispersions as disclosed in U.S. Pat. No. 4,092,168, EP 0 274 723 and EP 0 299 435.
In recent years new techniques of image recording have emerged wherein the exposure source of the recording apparatus is a laser unit. For instance, one application of lasers as exposure units is situated in the field of radiographic monitoring photography where a hard copy of radiographic information has to be produced. Other applications are situated in the pre-press field of phototype-setting and image-setting, where recorders are marketed which employ Ar ion lasers, He-Ne lasers or solid state semiconductor lasers, also called laserdiodes, as exposure source. The latter type of laser, the laserdiode, shows some advantages compared to other laser types such as low cost price, small size, long life time and no need for an acoustic-optical modulator. Generally the emission wavelength of these semiconductor laser beams is longer than 700 nm and mostly longer than 750 nm. So photographic materials appropriate for exposure by devices employing such laserdiodes must be sensitized for the near infra-red region of the radiation spectrum. Infra-red sensitized photographic materials are known for quite some time in photographic technology. One of the well-known early applications was and still is in aerial photography. Suitable photographic materials to be used with a semiconductor laser device are disclosed in Japanese Unexamined Patent Publication (Kokai) No 61752/85 and U.S. Pat. No. 4,784,933. Commercial infra-red sensitized film and paper were announced by Eastman Kodak Co in "Proceedings Lasers in Graphics", Electronic Publishing in the '80's, Vol 2 (Sept. 1985) p. 508, and put on the market lateron. Other manufacturers followed including Agfa-Gevaert N. V. and Fuji Photo Film Co. Examples of image-setters using a laserdiode exposure are the PROSET 9800 and the ELAN apparatus marketed by Bayer Co., Agfa division.
An infra-red sensitized photothermographic material for medical imaging purposes and based on Dry Silver technology is marketed by 3M under the trade name Dry View.
An infra-red sensitized element must contain an infra-red spectral sensitizer (see references furtheron) but it also preferably contains one or more non-sensitizing infra-red absorbing dyes for use as filter-, acutance- and/or antihalation dyes.
A suitable non-sensitizing infra-red dye can be chosen from the numerous disclosures and patent applications in the field, e.g., from U.S. Pat. No's 4,886,733, 5,075,205, 5,077,186, 5,153,112, 5,244,771, from Japanese unexamined patent publications (Kokai) No.'s 01-253734, 01-253735, 01-253736, 01-293343, 01-234844, 02-3037, 02-4244, 02-127638, 01-227148, 02-165133, 02-110451, 02-234157, 02-223944, 02-108040, 02-259753, 02-187751, 02-68544, 02-167538, 02-201351, 02-201352, 03-23441, 03-10240, 03-10239, 03-13937, 03-96942, 03-217837, 03-135553, 03-235940, and from the European published patent applications publ. No.'s 0 483 740, 0 502 508, 0 523 465, 0 539 786, 0 539 978, 0 530 511 and 0 568 022, and from International Patent publication WO 95/07822. This list is far from exhaustive and limited to rather recent disclosures. Older disclosures on infra-red absorbing dyes include e.g. EP 0 251 282, in EP 0 288 076, and in U.S. Pat. No. 4,839,265.
A large part of the patents on infra-red dyes cited above include heptamethine cyanines with indolenine nuclei and penta- and heptamethine cyanines with quinoline nuclei that contain sulphonic acid groups or salts thereof as water-solubilizing groups. These compounds are excellent infra-red antihalation dyes but the sulphonic acid substituents make them diffusible under normal coating conditions. On the other hand it is known in the prior art that antihalation dyes (not limited to infra-red dyes) can be rendered resistant to diffusion by providing their molecular structure with a carboxylic acid group as solubilizing group. At neutral pH these dyes are present as non-diffusible solid particle dispersions, while they become soluble and diffusable under the influence of an alkaline developer. For instance, U.S. Pat. No. 5,075,205 discloses infra-red absorbing dyes of the indoaniline type bearing a --COOH group. JP-A 03-13937 describes solid particle dispersions of heptamethin cyanins with --COOH as solubilizing group. According to U.S. Pat. No. 4,940,654 other solubilizing groups can be used provided their pK.sub.a value is situated in the proper range.
The present invention extends the teachings on infra-red absorbing non-sensitizing dyes.
It is the object of the present invention to provide a new class of infra-red absorbing compounds that are suited as antihalation-, acutance-, or filter dyes in infra-red sensitized photographic- or photothermographic materials.
It is a further object of the present invention to provide infra-red absorbing dyes that can be incorporated as solid particle dispersions, resistant to diffusion at normal pH but becoming soluble and diffusable under alkaline developer conditions.
Further objects of the invention will become clear from the description hereinafter.